1. Field of the Invention
The present invention relates generally to ion exchange systems which remove undesirable components from liquid solutions. It relates more particularly to a unique method of configuring and operating an ion exchange system whereby the hydraulic capacity of the ion exchange system is increased and the efficiency of impurity adsorption is improved. Although the method of the present invention has many different applications, it is described herein primarily as used in the treatment of sweetener solutions manufactured in a corn wet milling plant.
2. Description of the Related Art
Ion exchange is the process of removing unwanted ions from a solution by transferring them to a solid material, called an ion exchange resin, which accepts them while returning to the solution an equivalent number of preferred ions stored on the ion exchange resin. Ion exchange systems which involve the passing of a liquid solution containing unwanted ions such as organics, hardness, alkalinity, iron and manganese through an ion exchange unit so as to purify the liquid are well known in the water conditioning, metal finishing and paper industries. Such processes are sometimes referred to as adsorber systems, and the process whereby dissolved constituents in the form of unwanted ions are separated from an aqueous solution by passage through an ion exchange resin or adsorber bed is sometimes referred to as adsorptive separation.
These processes generally involve passing an aqueous solution, the influent stream, which contains the undesirable constituents or impurities through ion exchange units which are configured in pairs and connected in series to each other. Each pair typically consists of one ion exchange unit which contains a cationic resin bed and a second ion exchange unit which contains an anionic resin bed. The cationic resin contains negatively charged sites and takes up positively charged ions from the solution. Anion exchangers have positively charged sites and, consequently, take up negative ions. As the influent stream passes through the first or primary ion exchange unit pair, undesirable constituents in the influent are attracted to and adsorbed onto the resin beds and thus are removed from the liquid being treated. As the solution leaves the primary pair and enters the second ion exchange unit pair, the concentration of the constituents to be removed and which are available to be adsorbed decreases by virtue of the removal of some of the constituents within the first ion exchanger pair. The resin bed within an ion exchange unit has a limited capacity for the storage of ions, called its exchange capacity; because of this, the resin eventually becomes depleted of its desirable ions and instead becomes saturated with unwanted ions. Therefore, as more and more of the solution passes in series first through the primary and then through the secondary ion exchanger unit pair, the exchange capacity of the resin within each pair becomes depleted or exhausted, thereby reducing its effectiveness at adsorbing the impurities. Because the concentration of these constituents is greatest when the influent contacts the primary ion exchange unit pair, it is the resin within the primary pair which typically becomes exhausted first.
To restore the effectiveness of the ion exchange operation, the resin bed within an ion exchange unit must be regenerated through a process step which removes the adsorbed constituent from the resin. The resin bed is generally washed with a strong regenerating solution containing the desirable species of ions, and these ions then replace the accumulated undesirable ions, returning the exchange material or resin to a useable condition. This operation is a cyclic chemical process and the complete cycle usually includes backwashing, regeneration, rinsing, and returning the exchanger to service. Typically, the ion exchanger pair to be regenerated is removed from service while it is being regenerated and a previously regenerated exchanger pair which was off-stream is brought back on-line. Thus, in a typical operation the primary pair of exchangers becomes exhausted first and are removed from service so they can be regenerated. The secondary pair of exchangers then moves into the primary pair position and the previously off-line regenerated ion exchanger pair comes back on-line in the secondary pair position where it is hydraulically connected, in series, to the exchangers now occupying the primary position.
U.S. Pat. No. 2,413,844 to Rawlings dated Jan. 7, 1947 discloses a process for the ion exchange treatment of sugar utilizing a method of operation of the kind referred to above. In the operation of the invention to Rawlings, three ion exchanger stations are configured in series. When the first ion exchanger resin bed has reached a predetermined degree of exhaustion, it is removed from the process stream and the next ion exchanger bed in the series, which is less exhausted, is made to operate in place of the first. The third bed, which is still less exhausted, is then caused to function in the second place while a new or regenerated bed is configured to operate in the third place. The system then continues to operate again until the exhaustion of the first bed necessitates a change. Thus, in a typical operation of the invention of Rawlings, the primary ion exchanger pair is removed from service to be regenerated. The secondary pair then moves into the primary pair position, the third pair moves into the secondary position, and a previously off-line regeneration pair moves into the third position.
U.S. Pat. No. 2,578,938 to Kunin et al. dated Dec. 18, 1951 discloses a process pertaining to the deionization of sugar solutions. Therein, a plurality of ion exchange beds are again operated in series with the first bed, after it becomes exhausted, being removed from service for regeneration and a fresh ion exchanger added to the end of the series.
Similarly, U.S. Pat. No. 4,968,353 to Kawasaki et al. dated Nov. 6, 1990 discloses an ion exchange refining system for sugar liquor comprising three types of adsorption units, called towers, which are operated in series. These three types of adsorption towers are used as a pre-stage adsorption tower, a post-stage adsorption tower and an adsorption tower for regeneration. The sugar liquor is continuously passed from the pre-stage adsorption tower to the post-stage adsorption tower. The lowering of the refining capacity of the post-stage adsorption tower is sensed to shift the pre-stage adsorption tower to the adsorption tower for regeneration, while simultaneously shifting the post-stage adsorption tower and the adsorption tower for regeneration to the pre-stage adsorption tower and to the post-stage adsorption tower, respectively. Thus, in the invention of Kawasaki, as in the invention of Rawlings, the ion exchange units, after their regeneration, are moved countercurrent to the influent flow, that is, from the third to the second to the primary position before once again being regenerated.
U.S. Pat. No. 5,116,511 to Green et al. dated May 26, 1992 discloses a water treatment system for removing metal ions from water using a plurality of ion exchange columns operated in series and a method for cleaning the system. In this invention at least two columns are used to purify water while a third column is being regenerated. When the first or lead column in the water purification process becomes saturated with metal ions it is removed from the process stream so it can be regenerated. The second column is advanced to the position of the first column and a previously-cleaned column is moved into the second position. Thus, the system of this invention operates by rotating columns between purification and cleaning stages.
Not all ion exchange systems are operated in series. For example, U.S. Pat. No. 5,073,255 to Chili et al. discloses a water treatment system apparatus having at least a first and second water treatment tank in parallel flow relation wherein the tanks are regenerated in an alternating, intermittent manner. That is, raw water in branched, parallel flow is passed through a plurality of water treatment tanks. As the process proceeds, at least one of the tanks is disconnected and taken out of service while it is automatically regenerated. During this process treated water is continuously available by means of the other tank(s) which remain in operation. Thereafter, the tank automatically regenerated is placed back into service and a second tank is automatically disconnected from service and regenerated. The process thus proceeds in a similar manner. Thus a water treatment apparatus is provided in which multiple tanks, in parallel flow relation, may be automatically regenerated.
The invention of Brane et al. (U.S. Pat. No. 5,300,230) dated Apr. 5, 1994 also discloses a method for operating a water treatment system having a plurality of ion exchange units connected in parallel flow relation which are cyclically regenerated.
The invention of Brown et al. (U.S. Pat. No. 5,069,779) dated Dec. 3, 1991 also discloses a water treatment system having multiple treatment tanks maintained in a parallel flow relationship and a control arrangement for regenerating the resin tanks without substantially reducing the flow rate through the treatment system during a regeneration cycle.
The present invention relates to the removal of impurities from sweetener solutions using an ion exchange system but configured and operated in a manner not known in the prior art.